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%I #13 Mar 18 2017 11:20:48
%S 7,9,11,13,14,18,21,22,23,25,26,28,29,33,35,36,39,41,42,44,45,46,47,
%T 50,52,53,55,56,58,59,65,66,69,70,72,75,78,82,83,84,87,88,89,90,92,94,
%U 97,100,104,105,106,107,110,112,113,115,116,118,119,123,130,132,137,138
%N Numbers n such that phi(n) = 2^k*prime for some k >= 0.
%C Sequence is infinite, since 2n is in the sequence if and only if n is in the sequence. What is its density? - _Charles R Greathouse IV_, Feb 21 2013
%H Charles R Greathouse IV, <a href="/A066669/b066669.txt">Table of n, a(n) for n = 1..10000</a>
%e 7 is here because phi(7)=6 divided by 2 is prime; 21 is here because phi(21)=12 divided by 4 is prime; 15 is not here because phi(15)=8 divided by 8 is not prime.
%t Select[Range@ 138, PrimeQ@ Last@ Most@ NestWhileList[#/2 &, EulerPhi@ #, IntegerQ@ # &] &] (* _Michael De Vlieger_, Mar 18 2017 *)
%o (PARI) is(n)=n=eulerphi(n);isprime(n>>valuation(n,2)) \\ _Charles R Greathouse IV_, Feb 21 2013
%Y Cf. A000010, A066670-A066673, A065966.
%K nonn
%O 1,1
%A _Labos Elemer_, Dec 18 2001
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